Microscopes provide imaging in two general modes, transmission mode and reflection mode. Transmission mode relates to illumination through a sample, where reflection mode relates to illumination that returns from the surface of the sample. Various historic microscope apparatus and method enabling both modes of microscopy rely on different types of physical interactions to provide magnified imaging of a sample.
For example, conventional optical microscopes rely upon light reflecting from, or transmitting through a sample. The light is then passed through focusing lenses or mirrors to achieve high magnification. The resolution limit of conventional optical microscopes is related to the light wavelength of hundreds of nanometers. Electron and ion microscopes rely on charged particles, focused using lenses that employ electric or magnetic fields, and offer significantly higher resolution potential due to the shorter deBroglie wavelength of the relatively high-Momentum particles used, in most cases under 0.1 nm. The deBroglie wavelength of a particle can be thought of as a scale of distance over which a particle interacts, similarly to the wavelength of light. This wavelength is, λ=h/m0V, where h is Planck's constant, m0 is the particle mass and V the particle velocity.
The Scanning Electron Microscope and Helium Ion Microscope rely upon a charged particle beam generally of 1,000 to 50,000 eV energy directed at the sample, as compared to 2 to 3 eV for visible light. Albeit the imaging resolution is extremely good, but the high kinetic energy and the charge of the particles directed at the sample can be destructive to and/or reactive with the sample. In addition, the resulting electrical charging of insulating samples can interfere with successful imaging. At such energies, the beam particles also penetrate many atomic layers through the sample and therefore do not exclusively image the surface atomic layer of the sample, but may instead produce images from some greater depth range through the sample, as is the case for conventional optical microscopes for wavelength reasons.
For over two decades scientists have pursued imaging surfaces using a focused beam of neutrally charged atoms or molecules. Imaging without high energy beams while achieving high resolution is theoretically possible this way, because the high mass of atoms compared to electrons produces a short deBroglie wavelength, under 0.1 nm even at less than 0.1 eV energy. Molecular beam experiments show that certain neutral atoms and molecules at this energy scatter from the top atomic layer of samples, and imaging with this method could result in new information about materials and objects that cannot be readily obtained using previous forms of microscopy.
However, previous attempts at a neutral particle microscope have produced poor image signal to noise ratio and/or poor resolution due to a combination of problems. One problem is the difficulty of finding a suitable focusing element able to produce a high intensity, sharply focused beam spot of neutral particles. Neutral atoms and molecules are not strongly affected by electric or magnetic fields, and for the most part, scatter randomly off of mirror surfaces, making it difficult to focus, control, and direct the beam for imaging purposes. A second problem is the poor sensitivity of available neutral atom or neutral molecule detectors, which can only detect a very small fraction of the particles entering them. Probably the first images published from a neutral atom microscope were published in 2008, and were obtained in transmission mode.1 They were of poor signal to noise ratio and had somewhat better than 2 μm resolution. Prior to the disclosed invention, no published images improved significantly on these and none were obtained in reflection mode. 1 Imaging with neutral atoms—a new matter-wave microscope, M. Koch, S. Rehbein, G. Schmahl, T. Reisinger, G. Bracco, W. E. Ernst, and B. Hoist., Journal of Microscopy 229: 1 (2008).
Accordingly a need exists for a microscope apparatus and method which provides reflective mode and transmission mode non-destructive imaging using neutral particles.